Mercaptan extraction process



Nov. 2, 1954 Filed Aug. 11, 1948 T. B. TOM ET AL 2,693,442 MERCAPTAN EXTRACTION PROCESS 3 Sheets-Sheet 1 do BHFLLVHBdWlL P; iNVENToRs:

Theodore 8. Tom 2 John 4.80/1 II.

VW M Attorney Nov. 2, 1954 Filed Aug. 11, 1948 3 Sheets-Sheet 2 NONHOMOGENEOUS SOLUTION VOLUMES OF XYLENOLS IN SOLUTION z Q 3 3 00 g w ll] 8 z o v I \9 N O 9 #8 3 8 8 9 Noun-10s IONVHLBW =10 sawmoA mmvrozes: N Theodore 8. Tom (35 John A. 80/! H u. il/W1 At/omey 1954 T. B. TOM ET AL MER CAPTAN EXTRACTION PROCESS 3 Sheets-Sheet 3 Filed Aug. 11, 1948 2,693,442 Patented Nov. 2, 1954 ice United; States Patent MERCAPTAN EXTRACTION PROCESS Theodore B. Tom, Hammond, Ind., and John A. Bolt, Chicago, Ill., assignors to Standard Oil Company, Chicago, 111., a corporation of Indiana Application August 11, 1948, Serial No. 43,688 4 Claims. (Cl. 196-82)- This invention relates to a process for separating mercaptans from relatively high boiling oils such as fuel oil distillates of intermediate boiling range. The invention relates particularly to a process for improving the burning characteristics of heater oil derived from pctroleum by distillation of crude' oils and cracked petroleum products. The invention is illustrated by drawings which show in Figures 1 and 2 graphical representations of data, and in Figure 3 a diagram of an apparatus suitable for carrying out the process. v p

In the refining of heater oils a serious problem has been encountered with formation of carbon or coke in oil burners, especially in domestic sleeve-type space to remove the. mercaptans because of their objectionable odor, a part or all of the mercaptans are converted to disulfides which have a considerably higher boiling range than that of the oil from which they are derived. Studies on burning performance have. shown that these high-boiling disulfides are particularly undesirable v in burning oils destined for use in critical-space heaters as shown below.

Fifty gallon quantities of several heater oils of varying disulfide content were burned in a Jungers model C sleeve-type space heater. In this test, conducted at a draft equivalent to 0.03 inch water pressure and at 0.3 gallon per hour flow rate, an oil producing 6 grams or less of carbon deposits on the heating surface in the course of a SO-gallon run is considered acceptable. The influence of high boiling disulfides can be seen in the table below:

Heater oil burning quality 7 Finished 011 Original o. 0 ar on Mercap- Dlsul- 011 Treatment tan No. fide lgegpnss- None 90 0 4. 0 Doctor Sweetened 0 89 20. 3 Solvent extracted to 7 0 7 5. 9

Mercaptan No., then doctor sweetened.

The mercaptan number, sometimes called the copper number, is the milligrams of mercaptan sulfur per 100 ml. of oil, generally determined by titration with a standardized copper salt solution. The disulfide number is the milligrams of disulfide sulfur per 100 ml. of oil. It is determined by reducing the disulfides (Zn-l-HCl) to mercaptans and then determining the mercaptan number. As demonstrated by the above data, it has become necessary either to remove the disulfides from the oil or prevent their formation in refining operations and even in storage where disulfides are produced by atmospheric oxidation of mercaptans. This invention is concerned with the first method of solving the problem of producing heater oils with satisfactory burning quality by extraction of the mercaptans from the oil before they have become oxidized to disulfides. I v

Numerous processes have been devised heretofore to extract mercaptans from petroleum oils, and particularly from light petroleum distillates such as gasoline and naphthas. The mercaptans found in these light distillates are quite reactive with caustic alkalies, and the lower molecular weight mercaptans, such as methyl and ethyl mercaptans, can be removed by washing with aqueous solutions of caustic alkalies, particularly sodium and potassium hydroxide. The removal of the mercaptans found in gasoline has been effected by extracting with caustic solutions in various solvents, particularly methanol, and the methanol-caustic process for removing mercaptans from gasoline has been developed on a commercial scale. However, when it was attempted to remove the heavy mercaptans from heater oils by this process, it was found that because of the higher oil solubility and/or the lower acidity of these heavy mercaptans, only partial separation of the mercaptans could be effected.

According to our invention, we have found that the heavy mercaptans occurring in heater oil can be extracted by a caustic alkali solution in methanol containing in addition to the methanol a substantial concentra tion of alkali metal salts of high molecular weight alkyl phenols, particularly the alkyl phenols of higher molecular weight than xylenol. It is not necessary to employ these heavy phenols in pure form and it is preferred to extract them from the heater oil itself by use of alkalies. It has heretofore been proposed to employ the phenols and cresols occurring in cracked gasoline as mercaptan solvents. We have found, however, that these substances are substantially less effective for extraction of the heavy mercaptans found in heater oils than are the heavy xylenols. The following distillation data show a comparison of the boiling ranges of the cresols obtained from a cracked gasoline and heavy xylenols obtained from a West Texas heater oil:

Distillation analysisASTM Heavy frgi n ah sr xylgnolts rom ea er olme on Initial Boiling Point 312 374 10% DistillecL. 385 400 50% Distilled 392 446 90% Dlstll1ed. 412 508 Maximum Boil g P 446 5 58 Referring to Figure 1 which is a graphical representation of these data, the upper line is the distillation curve for heater oil heavy xylenols and the lower curve is for gasoline cresols. It will be seen that the 90% point of cresols from gasoline is approximately the same as the 15% point of the heavy xylenols from heater oil. .It will also be observed that the heavy xylenols from heater oil boil substantially above the boiling point of ordinary unsubstituted xylenols which is approximately 420 F. This temperature is indicated on the curve by the arrow, showing that approximately of the heater oil xylenols have boiling points above the boiling point of simple xylenols. However true boiling point data indicate that somwhat more than 75%, that is, to of the heavy xylenols boil above the boiling range of simple xylenols.

The composition of the heavy alkyl phenols found in heater oil is quite complex. Besides the simple xylenols, it contains alkyl xylenols in which the aromatic ring is substituted by additional alkyl groups such as methyl, ethyl, propyl, butyl, etc, The alkyl groups may also be substituted in in place of one or both of the xylenol methyl groups, thus providing higher boiling phenols resembling the xylenols but not true xylenols. For convenience in describing our process, compounds of this type are also included in the class of heavy xylenols, this term signifying those phenols from petroleum distillates which boil with the xylenols and above. Specific gravities of heavy xylenols from the heater oil out of West Texas crude fall in the range of about 0.91 to 0.99, but in the case of heavy xylenols from other crudes specific gravities may be somewhat higher, for example 102. In most cases the specific gravities of the heavy xylenols are somewhat below the specific gravity of the phenols from gasoline.

In order to employ the alkali metal heavy xylenolates for extraction of mercaptans with caustic solutions, we have found that a substantial amount of methanol is necessary to hold the heavy xylenolates in solution. When employed in this way they are substantially more effective solvents for the heavy mercaptans of heater oil than are the lower molecular Weight cresylates and alkyl cresylates. The relatively low solubility of the heavy xylenols in strong caustic solution makes their use highly impracticable in ordinary water solutions of the caustic alkalies. The minimum amount of methanol required for producing and maintaining, after heater oil contact, a homogeneous solution of heavy xylenolates in 100 volumes of aqueous 54.7% KOH, i. e. saturated at 90 F., is shown in the graph of Figure 2. From this graph, it will be seen that higher concentrations of methanol are required when the concentration of the heavy xylenol is increased. According to our invention, we employ 2-20 volumes of the heavy xylenols per 100 volumes KOH solution with sufiicient methanol to insure a homogeneous solution, e. g. about 19 to 35 volumes. The use of greater amounts of methanol than the minimum required for this purpose gives satisfactory results in mercaptan extraction but is ordinarily disadvantageous from the economic standpoint because of the necessity of recovering the methanol and eifecting separation from mercaptans. If the amount of methanol is excessive, however, we have found that the efliciency of mercaptan separation gradually diminishes. Accordingly, we prefer to use not more than about 40 volumes of methanol per 100 volumes of aqueous concentrated caustic solution. We have also found that where the amount of xylenols in the solution is small, the amount of methanol required for efiicient extraction should be increased substantially above the minimum required for homogeneity. Expressed in percentage by weight based on the total solution, we prefer to operate in the range of about 1 to 12% xylenolates and about to 20% methanol, the remainder being caustic and water.

In general, we prefer to use KOH solutions instead of NaOH solutions. The amount of KOH present in the aqueous caustic portion of the solution should be in the range of about 35 to 60 per cent; less with NaOI-I, e. g. 30 to 50%. When preparing the extraction solution, it is preferred to dissolve the caustic alkali in water to the desired concentration, then add the methanol and heavvy xylenol or xylenolates. When operating at ordinary temperatures of 80 to 90 F. with caustic potash, the amount of KOH is preferably about 50 to 55 per cent. At higher temperatures, stronger caustic potash solutions can be employed without solidification, but temperatures above 100 F. cause some hydrolysis of mercaptides and hence are undesirable.

It is important to avoid contacting the solution with CO2, either in air or dissolved in water or oil, because of its effect in neutralizing a portion of the caustic alkali ivlilth formation of troublesome carbonates of low soluity.

The operation of the process may be observed by referring to Figure 3 of the drawing which illustrates the treatment of a mercaptan-containing heater 011 similar to that described herein. According to the drawing. heater oil is charged to the process by line 10 leading to mixer 11 and settler 12 in which an alkali solution is employed for the primary purpose of removing H25 and CO2. For this purpose a dilute solution of caustic soda, e. g. less than 10%, is satisfactory. It can be recycled by pump 13 from settler 12 to mixer 11 and discarded when spent.

The HzS-free heater oil is conducted by line 14 to mixer 15 and settler 16, the first mercaptan extraction stage. The extraction may be carried out in a baffie tower employing countercurrent flow of oil to caustic solution, but we have found that a satisfactory extraction can be obtained with only two to five stages and where KOH solutions are used only two to four stages are required. The drawing shows an operation with two extraction stages.

In mixer 15, caustic-methanol-heavy xylenolate solutions are introduced by lines 17 and 18, are mixed with the oil in 15 and settled in 16. The caustic solution from settler 16 can be recycled by pump 19 to mixer 15 to increase the volume of solution contacting the oil and obtain more etfective contact. fil 1.3m? 9 solution to oil may be about 0.5 to 1 up to 2 to 1, fresh solution from line 17 being about one-fifth the volume of the oil. Separated oil from the first extraction stage is conducted by line 20 to mixer 21 and settler 22, caustic solution being supplied to mixer 21 by line 23 and recycle line 24, separated caustic from settler 22 being recycled by pump 25 as before. Again the volume of solution introduced by line 23 is about 20% of the volume of the oil contacted. Separated oil from settler 22, substantially free of mercaptans, is conducted by line 26 to mixer 27 and settler 28 in which the oil is washed with water to recover a small amount of methanol dissolved therein. Because of the relatively high molecular weight of the heater oil, the amount of methanol soluble therein is quite small, usually less than about 2% when employing caustic-methanol heavy xylenolate solutions described hereinabove. Wash water is supplied to mixer 27 by line 29. Wash water from settler 28 can be recycled by pump 30 to mixer 27 to increase the volume of water for contacting the oil stream. Finished, substantially mercaptan-free heater oil product is withdrawn from settler 28 by line 31. A trace of mercaptan, corresponding to about 2 to 10 mercaptan number, remaining in the oil can be removed by air oxidation such as occurs when the product is allowed to stand in storage with access to air.

Inasmuch as heater oil is a relatively low-priced product and the refining operation involves the treatment of huge quantities, it is essential to recover the reagents employed in the process, specifically caustic alkali, methanol and heavy xylenolates. The recovery operation imposes numerous problems which We have solved in the manner described hereinafter. The rich caustic-methanol-heavy xylenolate solution, sometimes called the spent caustic solution, is withdrawn from the first extraction stage by line 32 leading to heat exchanger 33 and thence to heater 34 where it is heated to a temperature of about 210 F. The caustic-methanol solution then passes by line 37 to caustic stripper 38 in which methanol and mercaptans are substantially completely removed from the caustic solution and heavy xylenolates. Heat is supplied for reboiling the stripper by heater 39 and steam or water are introduced at a low point in the stripper for aiding in removing the mercaptans from the caustic solution by hydrolyzing alkali mercaptides, superheated steam being preferred. The temperature of the caustic solution in the base of the stripper 38 is preferably of the order of 250 to 350 F. Heater 39 can be employed solely for producing steam by shutting off the flow of caustic solution thereto. Water converted to steam therein serves to aid the removal of heavy mercaptans from the caustic solution, sufiicient heat being supplied by heater 39 to superheat the steam, preferably to a temperature of about 400 to 600 F.

From the top of stripper 38 vapors of methanol, water and substantially all the mercaptans are withdrawn by line 41 leading to condenser 42 in which the water, methanol and mercaptans are largely condensed. The heavy xylenolates remain behind in the still residue with the strong caustic. Some of the mercaptans in excess of their solubility in the methanol-water solution are withdrawn by line 43 from drum 44 while the methanolwater solution is pumped by pump 45 through heater 46 into fractionator 47. Alternatively the vapors from 38 may be conducted, without condensation, by line 41a, directly to methanol tower 47, the introduction of hot vapors to the tower reducing or even eliminating the heat required for fractionating methanol therein.

Heat for reboiling the fractionator 47 is supplied indirectly by heater 48. From fractionator 47, vapors of methanol, substantially free of water, are conducted overhead to condenser 49 and thence to receiver $0, a portion of the methanol being returned by line 51 as reflux to column 47. Some mercaptans are carried over with the methanol, usually less than 1%, but these mercaptans, being of minimum molecular weight, combine readily with the caustic solution in the subequent extraction stages. It is desirable to conduct the fractionation in 47 to give substantially anhydrous methanol, inasmuch as water tends to bring over mercaptans by azeotropic distillation.

Methanol from receiver is conducted by line 52 through cooler 53 and thence by line 54 to the stage extraction system. From the bottom of caustic stripper 38 the solution of hot caustic is conducted by line 55 to drum 56, in which separation of the heavy xylenolates may be allowed to take place if desired.

Hot aqueous caustic solution containing between 50 and 70 per cent KOH and saturated With heavy xylenolates, is withdrawn by line 58 leading to exchanger 33 and thence through cooler 59 to line 23 and mixer 21 as hereinbefore described. For convenience in controlling the extraction solution composition, we prefer to maintain the concentration of KOH at about 55%. More concentrated solutions tend to solidify on cooling, particularly in heat exchanger 33. The rate of flow of caustic solution in recycle line 58 and methanol in line 52 is controlled to maintain the proper composition of the extraction solution. Water may also be injected by line 59a into the recycle solution in line 23 to regulate the composition and extraction efficiency in case the KOH content of the aqueous phase in separator 56 exceeds about 55%. When operating with higher KOH concentrations, difliculty with solidification in coolers 33 and 59 can be avoided by diluting the hot solution With methanol through line 5801.

Water and mercaptans, largely free of methanol, are withdrawn from fractionator 47 by line 60 leading to settler 61 in which an oily upper layer of mercaptans is separated and withdrawn by line 62. The water, substantially mercaptan-free but which may contain a small amount of methanol, e. g. 1 to 5%, is conducted by lines 63 and 29 to washer 27, any excess water being withdrawn from the system by line 63a. A portion of the water controlled by valve 64 in line 65 may be diverted to the methanol-mercaptan stream entering separator 44 where it aids in effecting a rough preliminary separation of mercaptans from the methanol before charging to the methanol fractionator 47. Additional reagents required for maintaining the extraction solution in a high state of effectiveness can be added to the system from time to time to compensate for losses. Thus additional water can be added to drum 61 and additional caustic to drum 56, while methanol may be added to drum 50, or these components may be added through line 59a.

The following specification describes a typical heater oil refined by our process:

The results in the table below were obtained in the extraction of mercaptans from sour West Texas crude heater oil having a mercaptan number 62, by using a solution of caustic potash, methanol and heavy xylenols, with varying concentrations as indicated. The contact time was fifteen minutes at a temperature of 90 F., one volume of solution to four volumes of oil being used in two extraction stages.

Heater oil extractions Volumes of 54.7 aqueous KOH 100 100 100 100 100 100 100 Volumes of Methanol. 20 20 40 40 40 20 Volumes of Heavy Xylenols 2 5 40 0 1 Mercaptan Number:

After 151; Extraction... 12.0 8.0 7.0 6. 0 1 8.0 1 11.5 1 12.5 After 2nd Extraction... 5. 5 2. 5 1. 5 2.0 1 2. 5 2 9. 0 1 8. 0

1 Oil layer became colored. 1 Extremely low concentration of heavy xylenols gives poor extraction The above data indicate that the use of the heavy xylenolates in combination with sufficient methanol to retain the same in solution with the aqueous caustic is necessary to obtain efi'icient extraction of mercaptans to produce heater oil of satisfactory mercaptan number. The data also indicate that there is a maximum amount of heavy xylenolates which should be used, beyond which discoloration of the oil occurs. Moreover, if excessive amounts of xylenolates are incorporated in the mixed extractant, larger amounts of methanol than those designated above are required to insure homogeneity of the aqueous solution. The use of relatively small amounts of methanol is desirable from the standpoint of economical operation of our process.

Having thus described our invention, we claim:

1. A process for removing mercaptans from hydrocarbon oil heavier than gasoline which comprises countercurrently contacting said oil with a reagent solution comprising an aqueous solution of from about 30% to about 60% by weight of caustic, about 20 to about 40 volumes of methanol per 100 volumes of said aqueous solution and about 2 to about 20 volumes of heavy xylenols per 100 volumes of said aqueous solution, separating treated oil from the reagent solution, regenerating said reagent solution and thereby separating an overhead fraction comprising mercaptans, methanol and water and a bottoms fraction comprising caustic, xylenolates, and water, further separating mercaptans from methanol, further separating xylenolates from caustic-water solution, and recycling methanol, caustic and xylenolates for further use within the process in proportions as hereinbefore set forth.

2. A process for removing mercaptans from hydrocarbon oil heavier than gasoline which comprises countercurrently contacting said oil with a reagent solution comprising an aqueous solution of from about to about 53 Baum caustic, about 20 to about volumes of methanol per 100 volumes of said aqueous solution and about 2 to about 20 volumes of heavy xylenols per 100 volumes of said aqueous solution, separating treated oil from the reagent solution, regenerating said reagent solution and thereby separating an overhead fraction comprising mercaptans, methanol and water and a bottoms fraction comprising caustic, Xylenates, and water, further separating mercaptans from methanol, further separating xylenates from caustic-water solution, and recycling methanol, caustic and Xylenates for further use within the process in proportions as hereinbefore set forth.

3. The process of removing mercaptans from an oil without substantial color degradation of the oil which comprises intimately contacting a mercaptan containingoil boiling above the gasoline range with a reagent solution consisting essentially of (a) alkali metal hydroxide in sufiicient water to provide an aqueous caustic solution containing between about 30 and 60 weight percent caustic, (b) between about 20 and 40 volumes of methanol, per 100 volumes of aqueous caustic solution, and (c) between about 2 and 20 volumes of alkyl phenols of higher molecular weight than xylenol, per 100 volumes of aqueous caustic solution, at a temperature of between about 80 and 100 F., separating a substantially mercaptan-free oil from reagent solution contaminated with alkali metal mercaptides, removing mercaptans to regenerate said reagent solution and recycling said regenerated reagent solution to the oil contacting zone.

4. A sweetening process that does not substantially degrade the color of the oil, which comprises contacting a sour heater oil with between about and 200 volume percent, based on oil, of a reagent solution consisting essentially of (a) aqueous KOH solution containing between about 50 and weight percent of KOH, (b) between about 20 and 35 volumes of methanol per 100 volumes of aqueous KOH solution and (c) between about 5 and 20 volumes of heavy xylenols per 100 :Volumes of aqueous KOH solution, at a temperature be- ,tween about and F., for a time sufiicient to substantially sweeten said oil, separating a substantially sweet oil from a reagent solution phase, removing mercaptans from said separated reagent solution phase and recycling said regenerated reagent solution to the contacting zone and wherein said xylenols are obtained by caustic extraction of sour heater oil.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,202,039 Yabroff et a1. May 28, 1940 2,269,467 McCullough Jan. 13, 1942 2,316,753 Ayers et al Apr. 20, 1943 2,317,053 Henderson Apr. 20, 1943 2,428,623 Hewlett et al. Oct. 7, 1947 2,488,000 Bernard Nov. 15, 1949 

2. A PROCESS FOR REMOVING MERCAPTANS FROM HYDROCARBON OIL HEAVIER THAN GASOLINE WHICH COMPRISES COUNTERCURRENTLY CONTACTING SAID OIL WITH A REAGENT SOLUTION COMPRISING AN AQUEOUS SOLUTION OF FROM ABOUT 35* TO ABOUT 53* BAUME CAUSTIC, ABOUT 20 TO ABOUT 40 VOLUMES OF METHONOL PER 100 VOLUMES OF SAID AQUEOUS SOLUTION AND ABOUT 2 TO ABOUT 20 VOLUMES OF HEAVY XYLENOLS PER 100 VOLUMES OF SAID AQUEOUS SOLUTION, SEPARATING TREATED OIL FROM THE REAGENT SOLUTION, REGENERATING SAID REAGANT SOLUTION AND THEREBY SEPARATING AN OVERHEAD FRACTION COMPRISING MERCAPTANS, METHANOL AND WATER AND A BOTTOMS FRACTION COMPRISING CAUSTIC, XYLENATES, AND WATER, FURTHER SEPARATING MERCAPTANS FROM METHANOL, FURTHER SEPARATING XYLENATES FROM CAUSTIC-WATER SOLUTION, AND RECYCLING METHANOL, CAUSTIC AND ZYLENATES FOR FURTHER USE WITHIN THE PROCESS IN PROPORTIONS AS HEREINBEFORE SET FORTH. 